Protostellar evolution of intermediate-mass stars: Implications for the survival of fossil magnetic fields

The detection of strong magnetic fields in radiative stars, particularly in chemically peculiar Ap and Bp stars, has led to different theoretical scenarios regarding their origin, including the fossil-field hypothesis. However, field survival intrinsically depends on stellar structure. Studying the convective-to-radiative transition is essential to assess whether fossil magnetic fields can persist throughout protostellar evolution. We determine the characteristic mass of protostars at which the transition from a convective to a radiative state occurs, as a predominantly radiative structure during accretion may favor magnetic field survival. We studied the pre-main-sequence evolution of intermediate-mass protostars from convective initial models using the Modules for Experiments in Stellar Astrophysics 1D code, investigating the dependence on metallicity, average accretion rate, and accretion history. The transition mass from a predominantly convective to a radiative state depends on both average accretion rate and metallicity, occurring between sim1.3,M_⊙ and 2.8,M_⊙ for our 1,M_⊙ reference models (up to sim4,M_⊙ for other initial masses at the highest accretion rates). For the adopted polytropic initial models and imposed accretion histories, the accretion history has a relatively limited impact. Lower metallicity shifts the transition to lower masses. For metallicities of Z ,M_⊙, ^ and accretion rates above 4̧dot10^ -6 yr -1 , several models retain a convective outer envelope until the stopping point, where accreted magnetic fields may be subject to dissipation. Since the transition occurs between 1.3,M_⊙ and 2.8,M_⊙ for reference models, our results, within the adopted polytropic framework, provide a possible explanation for the preferential survival of magnetic fields in more massive stars (consistent with observations), provided that the radiative structure persists after accretion ceases; we did not follow such a phase. At high accretion rates, magnetic fields advected with the flow may still dissipate within outer convective layers. We predict an increased fraction of strongly magnetized radiative stars at metallicities below Z=0.01.